The present invention relates to fiber-optic communications networks, and more particularly, to distributed Raman-pumped optical amplifier systems with transient control capabilities for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. Amplifiers have also been studied that use diode-laser pumping to generate gain through stimulated Raman scattering. Optical amplifiers based on erbium-doped fibers and Raman pumping increase the strength of the optical signals being transmitted over the fiber-optic links. A distributed Raman amplifier is a Raman amplifier in which optical amplification occurs in a span of Raman-pumped transmission fiber.
Sometimes channels in a communications link may be abruptly added or dropped. Channels may be dropped due to an accidental fiber cut. Channels may also be added or dropped suddenly due to a network reconfiguration. When the number of channels carried by a transmission fiber span changes abruptly, the total signal power being transported over the span changes suddenly. If a distributed Raman amplifier is pumped at a constant power, these sudden changes in signal power will result in transient effects in the gain of the distributed Raman amplifier. Gain transients in a distributed Raman amplifier may cause fluctuations in the power of the output signals from the amplifier. Output signals that are too weak may be difficult to detect without errors. Output signals that are too strong may give rise to nonlinear optical effects in fiber.
It is an object of the present invention to provide distributed Raman-pumped optical amplifier systems in which gain transients are controlled.
This and other objects of the invention are accomplished in accordance with the present invention by providing distributed Raman amplifier systems in which gain transients are controlled. Signal taps may be used to monitor optical signal powers at network nodes. A telemetry channel may be used to share information between nodes. Information on the output power of a given node may be passed to a subsequent node using the telemetry channel. The subsequent node may use feed-forward and feedback control schemes to control Raman gain transients in the preceding transmission fiber span based on the output power information received over the telemetry channel.
Each node in an optical communications link may include a rare-earth-doped fiber amplifier stage or other suitable amplifier for further amplifying optical signals that have been amplified by a distributed Raman amplifier. For example, erbium-doped fiber amplifiers may be used to amplify optical data signals before transmission to the next node in the link. Control electronics for erbium-doped fiber amplifier stages may be integrated with control electronics for distributed Raman amplifiers.
Signal taps in the erbium-doped fiber amplifier stages may be used to monitor input and output powers. Gain transients may be controlled in these fiber amplifier stages using feed-forward and feedback techniques.
Spectrum filters may be used for the power taps in the system to facilitate the use of feed-forward and feedback control techniques. For example, a spectrum filter may be used at the output of a given node. The spectrum filter may be used to impress the Raman gain spectrum of the following Raman-pumped transmission fiber span onto the measured output signal. Arrangements such as these allow feed-forward and feedback approaches to be used to control the Raman pump power for the span, even though the Raman gain spectrum is not flat. A combination of feed-forward and feedback techniques may be used to control amplifier transients in the distributed Raman amplifier if desired.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.